It's not entirely correct that the government "chose Tuscan" as the language to push. The literary tradition was already rooted into a vulgata that happened to be mostly similar to the languages spoken in the areas between Roma and Firenze - unsurprisingly so, considering they had traditionally been the wealthiest parts of the country for centuries. In this context of broad intellectual agreement on the fundamentals, Alessandro Manzoni then published a few works that explicitly tried to formalize the language, sprinkling northern inflections on top of the traditional core. These works were later used as the model by authorities, who forced them on the national curriculum.